The present invention relates to an optical fiber structure with modal propagation characteristics tailored to facilitate the fabrication of narrowband multimode fiber Bragg gratings, and more particularly the present invention relates to multimode optical fibers having concentric shells of alternating high and low refractive indexes.
Fiber grating structures in single mode fiber (SMF) have been studied extensively since the discovery of photosensitivity in germanium doped silica fiber. They have been used in numerous applications ranging from wavelength selective filtering in wavelength division multiple access (WDMA) systems, to temperature and strain sensing. To a lesser extent, Bragg gratings in multimode fibers have also received attention. Wanser et al. [1] calculated the theoretical spectrum of multimode fiber Bragg gratings (MMFG""s) and suggested their use for mircobend sensing. Mizunami et al. [2] experimentally confirmed the spectral properties of MMFG""s. A grating was fabricated in a graded index fiber that had a reflection spectrum of 15 nm width centered at 1560 nm, contained multiple peaks, and had a minimum transmission of 3.4% with selective mode launching. The reflection response was not measured. This same group later reported a detailed analysis of MMFG [3], including temperature and polarization characteristics. MMFG""s are also useful for tuning solid-state lasers in which the high coupling efficiency of multimode fiber (MMF) is attractive [4].
While the wideband, low reflectivity gratings that are possible in standard multimode fiber are useful, a narrowband, high reflectivity response could have a dramatic impact on optical communication systems. MMF""s have the advantage of easy coupling to inexpensive light sources such as light-emitting diodes (LED""s). In particular, graded index MMF""s have relatively low modal dispersion. These two factors in conjunction have led to the predominant use of graded index MMF in local area network applications. The possibility of a narrowband fiber Bragg grating in a MMF would enable the use of WDM in LAN""s. Other applications that would benefit include the field of in vivo Raman spectroscopy in which MMF is used for light collection efficiency, but narrowband filtering is required for signal recovery.
Therefore it would be advantageous to provide a multimode fiber having propagation characteristics tailored to facilitate the fabrication of narrowband multimode fiber Bragg gratings.
The present invention provides a novel multimode fiber structure with modal propagation characteristics tailored to facilitate the production of narrow-band, high reflectivity, fiber Bragg gratings.
In one aspect the present invention provides a multimode optical fiber, comprising:
a cylindrical fiber core and a plurality of concentric shells, the cylindrical fiber core and said plurality of concentric shells comprised of alternating higher and lower refractive index materials with said cylindrical fiber core comprised of said lower refractive index material, said higher refractive index material having a refractive index nh and said lower refractive index material having a refractive index nL, said higher refractive index shells having a thickness t, said higher and lower refractive indexes and said higher refractive index shell thickness t satisfying a relationship given by t(nh2xe2x88x92nL2)1/2 less than xcex/2, wherein xcex is a wavelength of light propagating in said optical fiber.
In this aspect of the invention the concentric shells having the lower refractive index nL may have a thickness xcex94, with xcex94 greater than 5xcex.In this aspect the core may have a radius of about 25xcex, wherein xcex is the vacuum wavelength, and xcex94 may be at least 10xcex.
In this aspect of the invention each of said high refractive index shells may include a grating written therein, each grating comprising a selected refractive index modulation along a length of the fiber. These gratings may include apodized gratings, chirped gratings, chirped Moirxc3xa9 gratings and long period gratings.
In another aspect of the present invention there is provided a multimode optical fiber, comprising:
a cylindrical fiber core and a plurality of concentric shells, the cylindrical fiber core and said plurality of concentric shells comprised of alternating higher and lower refractive index materials with said cylindrical fiber core being comprised of the higher refractive index material, said higher refractive index material having a refractive index nh and said lower refractive index material having a refractive index nL, said cylindrical fiber core having a radius R and the first concentric shell enveloping said cylindrical fiber core comprised of the low refractive index material having a wall thickness D, said higher and lower refractive indexes and said higher refractive index fiber core of radius R satisfying a relationship given by R(nh2xe2x88x92nL2)1/2 less than 0.383xcex, wherein xcex is the minimum wavelength which propagates in said higher refractive index core and concentric shells.
In this aspect of the invention each of said high refractive index shells may include a grating written therein, each grating comprising a selected refractive index modulation along a length of the fiber. These gratings may include apodized gratings, chirped gratings, chirped Moirxc3xa9 gratings and long period gratings.
In another aspect of the present invention there is provided a multimode optical fiber, comprising:
a cylindrical fiber core and at least one concentric shell having a thickness t, the cylindrical fiber core and said at least one concentric shell comprised of alternating lower and higher refractive index materials with said cylindrical fiber core comprised of said lower refractive index material and said at least one concentric shell comprised of said higher refractive index material, said higher refractive index material having a refractive index nh and said lower refractive index material having a refractive index nL, said higher and lower refractive indexes and said higher refractive index shell thickness t satisfying a relationship given by t(nh2xe2x88x92nL2)1/2 less than xcex/2, wherein xcex is a wavelength of light propagating in said optical fiber.